Conventionally, there has been known a side reinforcing type run-flat tire in which a reinforce rubber layer is arranged at a sidewall portion. In accordance with the run-flat tire mentioned above, the reinforce rubber layer supports a tire so as to suppress a flattening at a time when an air pressure in an inner portion of the tire is lowered due to a puncture or the like, whereby a run-flat traveling can be executed. In this case, since a pressure application to a rim of a bead portion is weakened in a state in which the air pressure in the inner portion of the tire is lowered (in a run-flat state), a fitting force to the rim is lowered, there is a problem that the bead portion tends to be detached from the rim.
In response thereto, in the following patent documents 1 and 2, there is disclosed a so-called double bead type run-flat tire provided with a first bead arranged in an outer circumferential side of a rim base, and a second bead arranged in an annular inflating portion inflating to an outer side in a tire width direction of a bead portion. In accordance with the run-flat tire mentioned above, since the annular inflating portion reinforced by the second bead is pressed to an outer circumferential curved side face of a rim flange at a time of a run-flat traveling, a fitting force to the rim is increased, and it is possible to improve a rim unseating resistance.
Further, in the conventional double bead type run-flat tire, the second bead also exists in an inner side of the vehicle, however, the second bead in the inner side of the vehicle has a small effect to the bead unseating, and an increase of a tire mass, an increase of a rolling resistance and a reduction of a riding comfortableness are caused by employing the symmetrical double bead structure, additionally.
On the other hand, in the case that the double bead structure is employed in the outer side of the vehicle, and the double bead structure is not employed in the inner side of the vehicle, a difference of deflection amount in the side portions in both sides becomes extremely large at a time of the run-flat. Accordingly, since an asymmetry characteristic of a ground contact pressure distribution of a tread surface becomes extremely large, and there is generated a partial-abrasion and a problem that a drivability is lowered, there has been no example in which the double bead structure is employed only in the outer side of the vehicle.
In this case, the following patent document 3 discloses a pneumatic tire in which a groove area in an outer side of a vehicle than an equator line of a tread is smaller than a groove area in an inner side of the vehicle (a void rate is smaller), and a rubber hardness in the outer side of the vehicle is larger than a rubber hardness in the inner side of the vehicle, however, does not teach a solving means in a technique in which the double bead structure is employed only in the outer side of the vehicle.
Patent Document 1: Japanese Unexamined Patent Publication No. S51-116507
Patent Document 2: Japanese Unexamined Patent Publication No. S53-138106
Patent Document 3: Japanese Unexamined Patent Publication No. 2003-326917